Such monoliths may be formed of a brittle fireproof ceramic material such as aluminum oxide, silicon oxide, magnesium oxide, zircon silicate, cordierite or silicon carbide and the like. These ceramic materials provide a skeleton type of structure with a plurality of tiny flow channels. Small shockloads are sufficient to crack or crush the monolith. Due to this brittleness problem which exists when using this type of catalytic device in connection with motor vehicles in which the ceramic monolith is located in a housing connected to the exhaust gas system, much effort has been expended in developing means for support of the monolith in its housing so that the monolith would be substantially free of shockloads. Representative of these efforts are the following:
U.S. Pat. No. 3,798,006 discloses securement of a monolithic type catalyst element in its housing by a differentially hardened fibrous lining. The monolith is supported by a felted layer or sleeve of ceramic fibers which are compressed between the monolith and a shell. After assembly, a suitable rigidizer, binder and adhesive liquid containing a high temperature-resistant material such as aqueous colloidal silica is applied to the compressed layer of ceramic fiber material. The treated unit is thereafter dried in a manner so as to cause migration of the silica solids to the exposed ends of the sleeve of ceramic fiber.
U.S. Pat. No. 3,876,384 discloses a monolithic catalyst carrier body which is resiliently mounted in a reactor casing by surrounding the monolith with a protective jacket which includes highly heat-resistant steel reinforcing means embedded in ceramic fiber and binder means, which itself includes a fireproof mortar. The monolith is enveloped by a protective jacket comprising an inner layer and outer layer of ceramic or mineral fibers which are embedded in a heat-resistant mortar. Steel reinforcing strips are embedded between the ceramic fiber layers and grip both of the ceramic fiber layers.
U.S. Pat. No. 3,891,396 discloses an elastic holder for monolithic catalyst bodies. The holder consists of a metallic corrugated tube which simultaneously forms the outer wall of the exhaust conduit. This corrugated tube is provided with a mechanical bias which safely holds the monolithic catalyst body and presses it against an end bearing. The monolithic body may be surrounded at its outer surface with elastic heat-resistant material in the form of ceramic wadding disposed in the space between the corrugated tube and the catalyst body or its ceramic sleeve. The catalyst body may be cemented with a heat-resistant cement to a ceramic sleeve which serves to thermally insulate the corrugated tube from hot exhaust gases.
U.S. Pat. No. 3,916,057 discloses a process for mounting monolithic catalyst support elements which utilizes an intumescent sheet material containing vermiculite or other expandable mica. The intumescent sheet material functions as a resilient mounting material by expansion in situ. The thermal stability and resilience of the sheet after exfoliation compensate for the difference in thermal expansion of the metal canister and the monolith and absorbs mechanical vibrations transmitted to the fragile monolith or forces which would otherwise be imposed on the monolith due to irregularities in the metallic or ceramic surfaces.
U.S. Pat. No. 4,048,363 discloses an offset laminated intumescent mounting mat for use in wrapping a ceramic catalytic monolith. After heating, expansion of the intumescent material in the mat secures the monolith in its housing or covering.
U.S. Pat. No. 4,142,864 discloses mounting of a catalytic ceramic monolith by positioning a resilient, flexible ceramic fiber mat or blanket in the space between the catalytic monolith and the inner surface of the casing. This blanket is compressed upon installation of annular plug members which are inserted at each end of the ceramic monolith between it and the casing. The plugs may be formed of solid metal, wire mesh or hollow metal.
U.S. Pat. Nos. 4,239,733 and 4,256,700 disclose a catalyst coated ceramic monolith supported in a sheet metal housing by both a wire mesh sleeve and an intumescent sleeve which are positioned adjacent each other in non-overlapping fashion.
U.S. Pat. No. 4,269,807 discloses a resilient mounting for a ceramic catalytic monolith in which the monolith is surrounded with a blanket of knitted wire mesh which is partially compressed throughout its length. Overlying the knitted wire mesh is a band of high-temperature intumescent material containing ceramic fibers as a viscous caulking or paste within the matrix of the metal mesh. Among the constructions disclosed is one which includes machining the ceramic monolith to remove 1/8 inch from its diameter and coating it with ceramic fibers of the corresponding thickness followed by surrounding with a blanket of knitted wire mesh.
U.S. Pat. No. 4,305,992 discloses flexible intumescent sheet materials suitable for use in mounting autmotive catalytic converter monoliths. These materials contain unexpanded ammonium ion-exchanged vermiculite flakes.
U.S. Pat. No. 4,328,187 discloses an elastic holder for axial suspension of a ceramic catalytic monolith within a housing. The monolith is surrounded with a layer of heat-resistant mineral fiber material. Overlying this fiber layer is a jacket or sleeve of good heat-insulating mineral material. Overlying the sleeve is a layer made from a highly-elastic material such as foam, asbestos or glass fiber fleece, or from a metallic wire mesh cushion which serves as a damping element which extends within the housing over the entire length of the monolith and elastically suspends the monolith together with its ceramic fiber wrapping and sleeve against the walls of the housing.
U.S. Pat. No. 4,335,077 discloses support of a ceramic catalytic monolith by means of elastically deformable damping rings or envelopes. In one embodiment the monolith is surrounded by a protective jacket of heat-resistant cement or putty reinforced with ceramic fibers. This protective jacket may be reinforced with metal in the form of a wire mesh or the like. The protective jacket is enveloped around its circumference by a soft mineral fiber layer which is compressed between the housing wall and the protective jacket.
U.S. Pat. No. 4,353,872 discloses support of a ceramic catalytic monolith within its casing by means of a gas seal member formed of heat-resistant and expandable sheet material, for example, vermiculite, quartz or asbestos, which envelopes a portion of the monolith. Longitudinally displaced therefrom is a separate layer of generally cylindrically knitted wire or resilient support which is disposed between the monolith and its casing to dampen external forces applied to the monolith.
U.S. Pat. No. 4,425,304 discloses a catalytic converter in which ceramic catalytic monoliths are supported by an elastic pad of expanded metal or steel mesh fabrics or a knitted web of ceramic fibers at their ends and are wrapped with respective cushioning layers of expanded metal or any other known flame-retardant, corrosion-resistant cushioning material.
U.S. Pat. No. 4,432,943 discloses an elastic suspension for a monolithic catalyst body in which the annular space between the housing and the catalyst body is filled with heat-resistant mineral fiber material which serves to prevent bypass of exhaust gas and as thermal insulation. In another construction the monolith is surrounded by a mineral fiber layer and a rigid sleeve of heat-resistant metal is positioned over the mineral fiber layer. The annular space between the sleeve and the housing may be filled with ceramic fiber.
Many of the aforedescribed means for support of a ceramic catalytic monolith have been adopted commercially for use in connection with gasoline powered passenger automobiles. In this type of service, the maximum converter temperatures are generally under 1600.degree. F. When attempts have been made to secure the ceramic monolith utilizing materials such as those disclosed in U.S. Pat. Nos. 3,916,056 and 4,305,992 in vehicles having a higher gross vehicle weight (GVW), failures have occurred which are believed due to failure of known intumescent sheet materials. One mode of failure observed is fragmentation of the ceramic monolith, another mode of failure has been shredding of the intumescent sheet material and consequent plugging of the next monolith in sequence. Large passenger automobiles may utilize catalytic converters which include two ceramic monoliths. Vehicles of higher gross vehicle weight, e.g. trucks, may require four serially arranged monoliths. Because of their high GVW, the engines of such vehicles operate at a much higher percentage of their maxiumum output, a much greater percentage of their operating time, than do the engines in passenger automobiles. These operating conditions in heavier vehicles result in maximum catalytic converter temperatures of much greater than 1600.degree. F. Converter monolith temperatures of 2000.degree. F. are not uncommon and temperatures of 2500.degree. F. may be encountered.
A typical passenger automobile catalytic converter utilizes a ceramic monolith which is supported by intumescent sheet material like that described in U.S. Pat. Nos. 3,916,057 or 4,305,992, having a nominal thickness of 0.195 inch and a nominal density of 40 pcf. This material is compressed during installation of the ceramic monolith into its metallic shell to a nominal thickness of 0.130 inch and a nominal density of about 60 pounds per cubic foot (pcf). Such a construction does not withstand the higher operating temperatures often encountered in the operating cycle of a higher GVW vehicle such as a truck. To overcome these deficiencies, it has been suggested that the overall nominal thickness of the compressed installed intumescent layer be increased to about 0.24 inch and the nominal density be increased to about 65-70 pounds per cubic foot as installed. While this latter construction does not immediately fail, it has been found after operation for a period of time that the layer of intumescent sheet material adjacent the catalytic converter was totally degraded, thus giving rise to the possibility of such degraded layer shredding and plugging the next monolith in sequence or the degradation continuing until the pre-compression force is released and the monlith is free to bounce about within the shell and self-destruct due to mechanical shock.